1. Field of Invention
The present invention relates to a method of manufacturing a semiconductor device, particularly to a semiconductor device including a memory cell region and peripheral circuit region, the peripheral circuit region having a transistor capable of operating under low power voltage.
2. Description of the Related Art
Among semiconductor devices, a typical nonvolatile memory such as a NAND flash memory includes a memory cell and a peripheral circuit cell. The memory cell is driven by the peripheral circuit cell and is provided with a low-voltage transistor. The peripheral circuit cell, on the other hand, is generally provided with both types of high- and low-voltage transistors of P-type and N-type respectively.
The latest application of such semiconductor device contains therein a low-voltage power source. Such configuration allows combination of two types of low-voltage transistors to be provided in a single device, namely a high-threshold type powered by external source and a low-threshold type powered by internal source. The process of forming the low-voltage transistor in such case involves implantation of ions into a well formed in the semiconductor substrate in order to control the threshold voltage. A complimentary metal oxide semiconductor integrated circuit (CMOSIC) having a combination of transistors of varying threshold voltages is disclosed in JP-A-2000-323587.
The aforementioned patent publication discloses a method of manufacturing a semiconductor device in which an N-well and a P-well is created whereafter MOS transistors having different threshold voltages are formed in each well by performing less number of times of ion implantation than the number of types of different threshold voltages.
However, when forming transistors having different threshold voltages by using wells of identical conductive type, a separate ion implantation process is required after forming the wells in order to form the transistors having different threshold voltages. Such requirement renders process cycle reduction difficult.